In general, the wheel bearing apparatus can rotationally support a wheel hub, mounting a wheel, via double row rolling bearings and are classified as a driving wheel and a driven wheel. For structural reasons, an inner ring rotation type wheel bearing apparatus is used for the driving wheel. Both the inner ring rotation type and outer ring rotation type are used for the driven wheel. In general, the wheel bearing apparatus is classified as a so-called first, second, third and fourth generation type. In a first generation type, the wheel bearing has double row angular-contact ball bearings fit between the knuckle and the wheel hub. The second generation type has a body mounting flange or a wheel mounting flange integrally formed on the outer circumference of an outer member. The third generation type has one of inner raceway surfaces directly formed on the outer circumference of a wheel hub. The fourth generation type has inner raceway surfaces directly formed on the outer circumferences, respectively, of the wheel hub and the outer joint member of a constant velocity universal joint.
The wheel bearing apparatus is provided with seals to prevent leakage of grease contained within the bearing apparatus and entry of rain water or dust from the outside into the bearing apparatus. Recently, it is desirable for the bearing apparatus to have a long durability along with a tendency of maintenance-free operation in the automobile. Under many circumstances, it has been proved that sealing trouble in the bearing apparatus, due to entry of rain water or dust into the bearing, leads to failure rather than peeling or breakage of structural elements of the bearings. Accordingly, it is very important to improve the sealability of the bearing apparatus in order to extend its life.
Recently, governments have required improve fuel consumption in view of energy saving and polluting of the global environment. Accordingly, it is important to reduce the rotational torque of the wheel bearing apparatus. In the wheel bearing apparatus, the sliding resistance of the sealing apparatus is predominant over the rolling resistance of the rolling elements (balls or tapered rollers). Thus, it is possible to reduce the rotational torque of the wheel bearing apparatus by suppressing the sliding resistance of the sealing apparatus.
A variety of sealing apparatus that improves sealability and reduce the rotational torque has been proposed. One example is shown in FIG. 5. This inner-side sealing apparatus 51 includes a slinger 53 adapted to be fit onto an inner member 52. A seal main body 55 is adapted to be fit into an outer member 54. The slinger 53 includes a cylindrical fitting portion 53a adapted to be fit onto the inner member 52. A standing portion 53b extends radially outward from the fitting portion 53a. In addition, the seal main body 55 has a metal core 56 adapted to fit into the outer member 54. A sealing member 57 is secured on the metal core 56. The sealing member 57 also includes a main lip 57b, a sub-lip 57c and side lip 57a closely contacted with a slinger 53.
Sealing lubricant (i.e. sealing grease) 58 is contained in a sealed manner in spaces between the side lip 57a, the main lip 57b and the slinger as well as between the main lip 57b, the sub-lip 57c and the slinger. When the seal main body 55 is assembled to the slinger 53, the tip end of the main lip 57b is directed to the outside “B” of the bearing. The main lip 57b contacts the surface of the fitting portion 53a of the slinger 53. The tip end of the sub-lip 57c is directed toward the inside “A” of the bearing. The sub-lip 57c contacts with the surface of the fitting portion 53a of the slinger 53. This makes it possible to prevent bearing lubricant (i.e. bearing grease), contained in the bearing, from leaking outside “B” of the bearing. On the other hand, the tip end of the side lip 57a is directed radially outward. The side lip 57a contacts the surface of the standing portion 53b of the slinger 53. This makes it possible to prevent the entry of foreign material, such as muddy water, from the outside “B” to the inside “A” of the bearing.
The sealing grease 58 is contained in spaces between the side lip 57a, the main lip 57b and the slinger 53 as well as between the main lip 57b, the sub-lip 57c and the slinger 53. The sealing grease 58 has kinematic viscosity of its base oil of 20˜65 mm2/s, preferably 20˜25 mm2/s at 40° C. and worked penetration of 220˜280, preferably 240˜260. In this case, if the kinematic viscosity of base oil is less than 20 mm2/s and the worked penetration exceeds 280, the flowability of the sealing grease 58 will be high. Thus, the sealing grease 58 would leak from the sliding surfaces of the slinger 53. On the contrary, if the kinematic viscosity of base oil exceeds 65 mm2/s and the worked penetration is less than 220, the apparent viscosity of the sealing grease 58 will be high. Thus, the frictional torque of the sealing lips 58 would be undesirably increased. If the worked penetration exceeds 280, the softness of the sealing grease 58 will be high. Thus, the sealing grease 58 would leak from the sliding surfaces of the slinger 53.
In the sealing apparatus 51, the main lip 57b, sub-lip 57c and side lip 57a closely contact the slinger 53 fit onto the inner member 52. Thus, it is possible to prevent leakage of the bearing grease from the inside “A” of the bearing and entry of foreign matter, such as muddy water, to the inside “A” of the bearing from the outside “B”. In addition, the sealing grease 58 is contained in a sealing manner in spaces between the side lip 57a and the main lip 57b as well as between the main lip 57b and the sub-lip 57c. The sealing grease has the kinematic viscosity of its base oil of 20˜65 mm2/s at 40° C. and the worked penetration of 220˜280. Thus, it is possible to reduce the frictional torque and accordingly the rotational torque of the sealing apparatus 51 (see, JP 2008-25668 A).
However, it is important to prevent reduction of the endurance life of the bearing due to lubrication defects of the bearing grease sealed in the bearing. Although the sealing grease 58, sealed in the sealing apparatus 51, may be different from the bearing grease sealed in the bearing, sometimes quality degradation problems are caused by both the bearing grease and the sealing grease due to the mixture of them together. Thus, it is important to select the best characteristics and combinations of both the bearing grease and the sealing grease that do not cause the above described problems.